reservoir that supplies the bay area with clean waterandenergy. butwhat would it take to retore it to what it ones was? measure f is what it's called. cbs 5's grace lee tells us it's just a first step in what could be a long political road. >> reporter: voters are considering if they want to take a closer look at our energy and water alternatives if we were to drain hetch hetchy. it would be one of the most emotional issues on the ballot this november. water has buried the hetch hetchy valley since the early 1920's. it supplies san francisco with 85% of its drinking water. now there is a movement to drain the reservoir and restore it to look like its twin, yosemite valley, complete with green meadows, birds, an entire ecosystem restored. it sounds crazy this idea of draining something that's been around since the early 20's. do you think that's possible? >> totally doable. >> reporter: he believes that after just one or two years, we could restore the grassland. within 50 years, a new forested canopy would emerge. >> once the political process is complete, the restoration proces